In general, prior art television receivers have been characterized as either the "hot-chassis" or "cold-chassis" type. In a hot-chassis receiver the necessary receiver supply voltages are directly derived from the AC line. The receiver chassis serves as the ground return for the supply voltages and is therefore electrically coupled to the AC line. This arrangement has proven less than optimal for a number of reasons. In particular, unless the television cabinet and operator-accessible controls are electrically isolated from the receiver chassis, it is possible that someone coming in contact with either the cabinet or controls will be caught across the AC line and therefore exposed to the danger of an electrical shock. The lack of isolation between the AC line and the receiver chassis causes additional problems when peripheral equipments, such as video tape recorders, are coupled to the receiver. In order for such equipments to interface properly with the receiver, it is necessary that there be a common reference, or return, potential. However, it is likely that substantial differences will exist between the reference potential of the peripheral equipment and that of the receiver circuitry. This can give rise to possibly damaging currents between the disparate reference potentials.
The above and other related problems can be avoided in a cold-chassis receiver. By using an isolation transformer with a primary winding connected to the AC line and a secondary winding connected to the receiver's circuitry, it is possible to maintain electrically isolated returns. However, in order to supply the receiver's power requirements, the isolation transformer must necessarily be of substantial size and weight, and of proportionate expense. Furthermore, the input transformer tends to degrade the receiver's power consumption efficiency.
The above problems led to the development of what has been called the "Iso-Hot" chassis. The Iso-Hot chassis, as the term indicates, is in essence a hybrid of the hot- and cold- chassis. Typically, a high voltage supply derived from the AC line is used to provide the DC collector supply voltage for the horizontal deflection output transistor. The collector supply voltage is coupled through the primary of the flyback transformer to the horizontal output transistor. As is well known, the horizontal output transistor is used to supply an alternating current to the primary of the flyback transformer. This current in turn induces a relatively high voltage in the flyback secondary from which is derived the anode voltage for the receiver's picture tube. The essence of the Iso-Hot concept is to provide additional windings on the flyback transformer so as to develop the relatively low voltage supplies required by the remainder of the receiver's circuitry. These windings and associated supplies can then be returned to a ground, typically the receiver chassis, that is electrically isolated from the flyback primary and the horizontal output transistor. As a result, a large part of the receiver's circuitry can be electrically isolated from the AC line derived supply and ground return.
However, the Iso-Hot concept, as described above, is not without its drawbacks. For instance, because the horizontal oscillator interacts with many other portions of the receiver's circuitry, it is necessary that its ground return be identical to that of, for example, the video circuitry. In other words, in order to preserve the isolation afforded by the flyback secondary, the voltage supply required by the horizontal oscillator must also be derived from the flyback secondary and must be returned to chassis ground. However, the voltage across the flyback secondary is magnetically induced by the alternating current in its primary. The alternating current in the primary results from the switching action of that winding between a source of voltage and a ground return. The switching portion is provided by the horizontal output transistor. The horizontal output transistor is driven by a horizontal driver transistor, which is in turn driven by the horizontal oscillator. But the horizontal oscillator is powered by a voltage derived from the flyback secondary. That is, in order for the oscillator to operate there must be a voltage induced in the flyback secondary. However, no voltage can be induced in the secondary until the oscillator is operating. Consequently, an Iso-Hot receiver must include a startup voltage supply for the horizontal oscillator and, in order to maintain the advantages afforded by the Iso-Hot concept; this supply must be isolated from the AC line.
One straight forward method of accomplishing this is to derive the startup supply from the secondary of a transformer whose primary is coupled to the AC line; but this is precisely what the Iso-Hot chassis was designed to avoid, an input isolation transformer. Even though the power requirements, as well as its weight and cost, of the Iso-Hot input transformer should be substantially less than that of the cold- chassis transformer, it is nevertheless desirable to provide a superior method of developing a startup supply.
In addition, in order to prevent damage to the receiver as a result of excessive horizontal deflection voltages or currents, it has become customary to monitor either the horizontal output transistor supply voltage or the flyback primary voltage and to disable the horizontal oscillator in response to an over-voltage condition. However, this is not easily accomplished in an Iso-Hot receiver wherein the flyback and the horizontal output supply voltage are electrically isolated from the horizontal oscillator. Any conventional method of doing this would necessarily sacrifice this feature.